At the welding lines in manufacturing firms, efforts are being made to improve the total operating efficiency by increasing the speed of welding operation. An increasing number of firms are trying to introduce a high-speed welding. Aiming to secure molten weld metals for enough volume in high-speed welding, some wire feeders capable of supplying weld wire for a sufficient amount are already made available.
Meanwhile, a tandem arc welding process has also been introduced as a means of increasing the speed of welding taking advantage of the technology of the above high-capacity wire feeders. In a tandem arc welding system, an integrated 2-electrode welding torch or two single-electrode welding torches disposed separated to each other by a certain specific distance is provided at the tip end part of a welding robot or the like device. A certain specific action is performed at a certain specific speed under a certain control in accordance with a certain operation program for carrying out a welding operation. The operation program is provided based on an assumption that in a welding section the two electrodes each including its weld wire disposed penetrating through the electrode are orientated substantially on a welding line, in front and the rear arrangement.
Now, a tandem arc welding system is described in its outline formation and operation referring to FIG. 1. FIG. 1 shows the outline of mechanical structure of a tandem arc welding system having an integrated 2-electrode welding torch. Integrated 2-electrode welding torch 150 is mounted on a welding robot manipulator or the like working gear, which is not shown. The torch travels on the surface of welding object 160 along a certain specified line for welding. A device which puts the robot manipulator, etc. into operation is connected with control unit 120. Control unit 120 is connected with two welders, 130 and 140. Respective welders, 130 and 140, are provided with a weld wire feeder, which is not shown. Each weld wire feeder feeds a weld wire, not shown, to welding torch 150, viz. two weld wires are delivered to the torch. Within welding torch 150, each of the two weld wires is transferred penetrating through an electrode tip, not shown, respectively. The electrode tips are connected to output terminals of welder 130 and welder 140 via power cable 131 and power cable 142, respectively. Electric powers from welder 130 and welder 140 are supplied to the weld wires, respectively. Welding object 160 is connected to the ground terminals of welder 130 and welder 140 by way of grounding cable 132 and grounding cable 141. The arcs generated between the weld wires and welding object 160 form welding current circuits.
Control unit 120 houses in it an operation program and welding conditions. Control unit 120 controls action of the welding robot manipulator, etc. in accordance with the operation program. Furthermore, control unit 120 transfers, corresponding to the action, those instructions and parameters to welder 130 and welder 140 via control line 133 and control line 143. Welder 130 and welder 140 control their own weld wire feeders so that the weld wires are supplied for specified amount in accordance with the parameters received from control unit 120.
In this way, a tandem arc welding system provides a certain specific welding on a welding object 160 at a certain specific place.
Now in the following, description is made on how the tandem arc welding is carried out referring to FIG. 2. FIG. 2 illustrates a scene where a tandem arc welding is being carried out with an integrated 2-electrode welding torch, in the direction from the right to the left. As to the terminologies regarding the direction of welding, “fore-going” means that which is going in the front, while “hind-going” means that which is going in chase of the “fore-going”. In the inside of nozzle 210 of integrated 2-electrode welding torch 150 (ref. FIG. 1), there are two electrode tips, fore-going electrode tip 201 and hind-going electrode tip 202, disposed at a certain specific electrode-to-electrode distance. Fore-going electrode tip 201 is supplied with fore-going weld wire 203, whereas hind-going electrode tip 202 is supplied with hind-going weld wire 204.
Fore-going weld wire 203 gets the power from welder 130 (ref. FIG. 1) which is equipped with a welding power supply, not shown, dedicated to the fore-going electrode via fore-going electrode tip 201, and generates fore-going arc 205 between fore-going wire 203 and welding object 160. The heat of arcing melts fore-going wire 203 and welding object 160 to supply molten pool 207 with the molten metals. At the same time, hind-going weld wire 204 gets the power from welder 140 (ref. FIG. 1) which is equipped with a welding power supply, not shown, dedicated to the hind-going electrode via hind-going electrode tip 202, and generates hind-going arc 206 between hind-going weld wire 204 and welding object 160. The heat of arcing melts hind-going wire 204 and welding object 160 to supply molten pool 207 with molten metals. Fore-going wire 203 and hind-going wire 204 are supplied continuously while integrated 2-electrode welding torch 150 (ref. FIG. 1) is traveling at a certain specified speed. Thus, molten metal pool 207 moves ahead and welding bead 270 is formed behind it. A welding operation is conducted in this way.
In the tandem arc welding, where two arcs are generated in proximity, one arc can not help giving influence to the other arc. At the welding start section and at the welding end section, among other sections, the arcs are in transient stage and cause interference to each other. This readily brings the arcs into a state of instability, which calls for an appropriate control on them.
At the welding start section, there is a risk of poor welding, a damage on the tip due to interruption or turbulence of arching, a sputtering, etc. Whereas, at the welding end section, there is an esthetic risk of poor appearance of welding bead 270, in addition to the above-described risk items. Aiming to avoid these risks, new proposals have been made; which include, for example, a control sequence for the welding start and welding end. Patent Document 1 is an example of such proposals.
However, the conventional methods of preventing mutual interference between the two arcs do not function satisfactorily for stabilizing the arcs in the high-speed welding. Molten metal pool 207 constitutes a key factor of the arc instability. For example; since hind-going arc 206 is positioned to generate its arc towards molten metal pool 207, the arc is easily influenced by a disorder in molten metal pool 207. Molten metal pool 207 tends to flow behind under the influence of arcing power of fore-going arc 205 generated by fore-going weld wire 203. Arcing power of hind-going arc 206 generated by hind-going weld wire 204, however, tries to push it back towards the front. So, in order to provide a stabilized molten metal pool 207, a certain balance needs to be established between the two. Molten metal pool 207 and the arcs are in the mutually influential relationship.
Furthermore, in a high-speed welding, a change of welding speed brings the state of molten metal pool 207 into instability. As the results, the arc becomes to be instable, which quite often makes it difficult to accomplish a consistent welding. This phenomenon becomes significant especially at the starting and ending of a high-speed welding. At the welding start, molten metal pool 207 is pulled at high speed when the welding speed is raised sharply. This causes the drawback. In the high-speed operation, the weld wire supply needs to be increased accordingly; abrupt increase of the supply ill-affects a balance established between the supply and the melting. This can be another cause of the inconvenience. On the other hand, when ending a high-speed welding, the molten metal pool is disturbed by a sudden braking. In order to have a high-speed welding introduced successfully in normal production, these problems need to be solved.